libxcb: use Python requirement only on Linux

See Homebrew#69508.

Formula/libxcb.rb

@@ -25,14 +25,20 @@ class Libxcb < Formula
   end
 
   depends_on "pkg-config" => :build
-  # Use existing Python 3, to avoid a cyclic dependency on Linux:
-  # python3 -> tcl-tk -> libx11 -> libxcb -> python3
-  depends_on Python3Requirement => :build
   depends_on "xcb-proto" => :build
   depends_on "libpthread-stubs"
   depends_on "libxau"
   depends_on "libxdmcp"
 
+  on_macos do
+    depends_on "python@3.9" => :build
+  end
+  on_linux do
+    # Use an existing Python 3, to avoid a cyclic dependency on Linux:
+    # python3 -> tcl-tk -> libx11 -> libxcb -> python3
+    depends_on Python3Requirement => :build
+  end
+
   def install
     args = %W[
       --prefix=#{prefix}